You want to set up a secure, internal network within Google Cloud for database servers.

To ensure servers do not have any direct communication with the public internet, they must be configured without a public IP address.

VPC and Private Subnet: A Virtual Private Cloud (VPC) network provides the isolated, internal network structure. A subnet is the logical partition within the VPC.

Private IP Address: Assigning only a private IP address to the database servers ensures they can only communicate internally within the VPC (or connected on-premises networks) and cannot directly connect to or be connected from the public internet.

Extracts:

"Resources in a VPC network can be assigned two types of IP addresses: internal (private) and external (public). If a VM is not assigned an external IP address, it can only communicate internally with other resources in the VPC network..." (Source 6.1)

Option A and C involve assigning a public IP address, which violates the "no direct communication with the public internet" rule. Option D uses NAT to provide outbound internet connectivity, which also violates the requirement.